Block Directory: Use the block's title for alt text on block director…

…y plugin items. (#19263)

Merged pull request #19263 into master.

* Block Directory: Use the block's title to construct an alt text for the block's icon when they are using an image file.

Fixes:
https://github.com/WordPress/gutenberg/pull/16524/files#r324268820

* Add an accompanying translator comment to explain the placeholder.

packages/block-directory/src/components/downloadable-block-header/index.js

@@ -2,7 +2,7 @@
  * WordPress dependencies
  */
 import { Button } from '@wordpress/components';
-import { __ } from '@wordpress/i18n';
+import { __, sprintf } from '@wordpress/i18n';
 
 /**
  * Internal dependencies
@@ -15,7 +15,8 @@ function DownloadableBlockHeader( { icon, title, rating, ratingCount, onClick }
 		<div className="block-directory-downloadable-block-header__row">
 			{
 				icon.match( /\.(jpeg|jpg|gif|png)$/ ) !== null ?
-					<img src={ icon } alt="block icon" /> :
+					// translators: %s: Name of the plugin e.g: "Akismet".
+					<img src={ icon } alt={ sprintf( __( '%s block icon' ), title ) } /> :
 					<span >
 						<BlockIcon icon={ icon } showColors />
 					</span>